In this application the two principle sound producing items are road noise and engine noise. These are dealt with somewhat differently. However, in most cases the method of absorption is used. The engine compartment usually has pretty good noise blocking capabilities, so lining the compartment inside and out (where practical) will do an excellent job. The hood is a particularly important place to start. Factory material is usually not anywhere near adequate to do an acceptable soundproofing job. The hood must be removed and thoroughly cleaned before applying soundproofing!